The techniques for handling data have recently made such rapid progress that they have almost come to be generally used in our daily lives. As an example, large-scale mobile telephone systems have either reached a practical stage or are on the drawing board.
In a mobile telephone network, a plurality of zones each having a radio station allotted with communication channels in the number of subscribers make up a control zone. Each control zone has a radio channel control station for controlling the radio stations within the particular control zone, thus establishing radio channels with mobile devices, i.e., telephones carried on automobiles. The mobile devices connected through these channels are connected to the public telephone network through an automobile telephone exchange coordinating a plurality of the radio channel control stations for connection between and charging on subscribers.
In the mobile communication system including automobile telephones mentioned above, a clock signal for data is required to be extracted for receiving digital data such as dial numbers. The prior art systems use a PLL circuit (phase locked loop) including a VCO (voltage controlled oscillator) or an LC tuning circuit in order to extract an LC clock signal. These circuits, however, are not suitable for conversion into integrated circuitry, so that a digital PLL circuit has recently come into use. The digital PLL circuit is capable of generating a frequency sufficiently high as compared with the clock frequency and requires a frequency source of high stability. In other words, in the mobile communication system for automobile telephones or the like, the transmitted signal is often displaced in time by such a phenomenon as reflection on a high-rise building. In order to extract the clock signal following up this displacement, a frequency source for generating a reference frequency which is highly stable for high frequencies is required for accurate receiving process.